FIGS. 1, 2, and 3 illustrates a joint pin 3 for connecting a baton 5 of a nunchaku to a chain 1 via a bearing 2 mounted in an end of the baton 5. The joint pin 3 is provided at a predetermined position with an annular groove 31 about 10 mm in depth. The joint pin 3 has an outer diameter close to an inner bore 21 of the bearing 2, and could therefore be extended through the bore 21 of the bearing 2 to project from the end of the baton 5 to rotatably connect to an end of the chain 1. Steel balls 22 mounted in the inner bore 21 of the bearing 2 are engaged with the annular groove 31 on the joint pin 3, allowing the baton 5 to smoothly rotate relative to the joint pin 3 and the chain 1 when the baton 5 is thrown or otherwise manipulated. The above-structured nunchaku necessitates the steel balls 22 of the bearing 2 to have a high friction coefficient and considerably high frequency of use to therefore subject to wear, that constantly reduces an outer diameter of the steel balls 22 to cause gaps between the steel balls 22 and the annular groove 31 of the joint pin 3. The gaps gradually increase with the use of the nunchaku and the steel balls 22 would eventually fail to effectively engage with the joint pin 3 via the annular groove 31. Under this condition, it is very possible for the joint pin 3 to disengage from the bearing 2, resulting in unexpected and dangerous separation of the baton 5 from the chain 1 while it is thrown outward. It is therefore desirable to improve the conventional nunchaku to eliminate such problems.